The present invention relates to the casting of lead alloy parts for automotive storage batteries, and more particularly, to the casting of lead alloy battery grids for use therein.
During the manufacture of electric storage batteries, and more particularly during the manufacture of electric storage battery plates therefor, lattice-like grids must be produced in order to receive and hold lead oxide pastes which are pasted thereon prior to the assembly and formation of the battery. Prior art methods of producing battery grids have included various stamping and casting processes. See for example U.S. Pat. Nos. 1,524,610, 2,079,727, 2,881,105, 3,408,236, 3,629,388, 3,779,816 and 3,009,459.
Although the difficulties encountered in casting battery grids are somewhat similar to those encountered in casting other lead parts for automotive batteries, the particular configuration of automotive battery grids and their intended end use within the battery make these parts particularly difficult to cast. Automotive battery grids are relatively thin substantially planar structures which consist essentially of a lattice of thin lead struts. In order to produce a high quality automotive battery, it is extremely important that the thickness of these grids be uniform and that the grids be substantially planar since any dimensional irregularities in these grids may present substantial problems when these grids are pasted and stacked to form the battery elements of a single storage battery. Additionally, each of these automotive battery grids must be designed to create a positive engagement between the grid material and the lead oxide paste which is applied thereto after casting. For this and other reasons, it is therefore sometimes desired to impart to the surface of the battery grids a rough texture, which texture further causes problems which may be encountered during the casting process, such as releasing the grids from the mold following casting. Finally, the surface to volume ratio of the conventional battery grid makes the heat transfer relation between the battery grid material and the mold material a very sensitive one.
Although the prior art is replete with methods of battery grid casting, including some methods in which the molds or mold substrates are coated prior to casting, only one such prior art method, a cork flour spray method, has experienced substantial commercial success. See U.S. Pat. Nos. 3,709,459 and 3,779,816. This method, as commonly practiced by the battery industry, uses a suspension of cork dust and clay in water and sodium silicate which is sprayed on the mold periodically, perhaps as often as once every three hours. This mold spray is used to impart a texture to the lead and to provide a slight insulating barrier between the molten lead alloy and the cast iron mold. This barrier permits the lead to remain molten while it flows properly into all areas of the mold.
Unfortunately, this cork flour spray method suffers from the disadvantages that as successive castings are taken, the spray wears thin and the dimensions of the battery grids which are produced by the molds tend to vary. Consequently, a casting from a freshly sprayed mold will be slightly undersized, whereas a mold from which many castings have been made will produce slightly oversized battery grids. As mentioned above, since minor dimensional irregularities in battery grids will produce great variations in the overall size of battery stacks to be produced therefrom, depending on the number of battery plates per stack, dimensional irregularities such as those encountered as cork spray coatings wear thin are undesirable due to (1) the wide tolerances which must be accomodated in the remainder of the battery production process, (2) the lack of uniformity in the finished product and (3) the amount of lead which is wasted when oversized battery grids are produced. For a discussion of this problem with respect to the pasting process, see U.S. Pat. No. 3,249,981. In spite of these disadvantages, the advantages provided by the cork flour spray process, including the advantage of being able to vary the finished texture of the battery grids produced thereby by varying the cork particle size applied thereto, results in the general acceptance of this process in the battery industry.
The configuration of prior art battery grid molds and several prior art battery grid mold coatings are disclosed in U.S. Pat. No. 3,779,816 (titanium oxide) and U.S. Pat. No. 3,709,459 (silicum nitride).